storage API: handle NoTimeFrame exception
storage API: handle NoTimeFrame exception

If there's no time frame available, return an empty list.

Change-Id: Ia1918a2a6dd2e69999de6aeff47a18d3b388b411

stackforge/cloudkitty,openstack/cloudkitty,FNST-OpenStack/cloudkitty,openstack/cloudkitty,FNST-OpenStack/cloudkitty,stackforge/cloudkitty,muraliselva10/cloudkitty,muraliselva10/cloudkitty